1. Field of the Invention
This invention relates to a method of driving a recording medium such as an optical disk or a magneto-optical disk and, more particularly, to a method of detecting an sensor signal for a tracking servo mechanism and a focusing servo mechanism and amplifying the detected signal.
2. Description of the Prior Art
In an optical disk system or a magneto-optical disk system, usually light power of low intensity is provided for illumination when reproducing data, while light power of high intensity is provided for illumination when recording data. An error signal used for light spot tracking servo or focusing servo utilizes light reflected by or transmitted through a disk. Therefore, with a change in light power the error signal is also changed. To obtain a stable servo signal, therefore, the level of the error signal is desirably constant irrespective of even a great change in the light power. To meet this end, it has been in practice to switch the gain of a pre-amplifier for amplifying a detector output in stages, one at the time of the reproduction and the other at the time of the recording. A specific example of such a system is disclosed in Japanese Patent Laid-Open Sho 60-22746. The basic structure of the system will be described with reference to FIG. 5.
Referring to the FIG. 5, reference numeral 1 designates a photo-sensor, 2 a differential amplifier for an out-of-track signal system, 3 and 5 amplifiers for providing a tracking servo drive signal to an output terminal 6, 7 a de-focus signal system differential amplifier, 8 and 10 amplifiers for providing a focus servo drive signal to an output terminal 11, 12 and 13 a total reflection light system inversion amplifier and a filter, respectively, and 4 and 9 are transistors provided between filter 13 and amplifiers 3 and 5. With such a prior art system the output current of the photo-sensor 1 is converted by an amplifier 2 into a voltage, which is then passed through an amplifier 3, which is capable of switching of the gain to one of two values, thus providing an error signal. In this case, by switching the gain of the amplifier 3 at the time of reproduction or recording, the error signal level can be held in a predetermined range, and a stable tracking servo signal can be obtained.
A focus servo signal can also be obtained in a similar way.
The prior art optical disk system has the above simple structure because no consideration is paid to the dealing of various different recording media by the same system. This is so because in this case the ratio of the magnitude of the photo-sensor output dealt with between the case of reproduction and the case of recording is about 1:10 so that the output voltages of the amplifiers 2 and 7 do not become less than the amplifier error at the time of the reproduction, at which time the output is low, and can be held within the amplifier output permissible range at the time of the recording, at which time the output is high. However, when consideration is paid to the dealing of various different recording media by the same system, the light power used for reproduction, recording and deletion varies with the individual media, and also the light reflectivity of the medium surface varies. Therefore, the reflected light power, which is the product of the illumination light power and light reflectivity, varies greatly. Consequently, the ratio between the minimum and maximum values of the sensor output amounts to one versus several hundred. For this reason, if again not less than the amplifier error is provided at the time of the low output, the gain at the time of the high output exceeds the output range. On the other hand, if a gain in the normal output range is provided at the time of the high output, at the time of the low output the gain is less than the error so that it becomes impossible to take out any signal.